A well known semiconductor memory component is random access memory (RAM). RAM permits repeated read and write operations on its memory elements. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Examples of RAM devices include dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMS and SDRAMS also typically store data in capacitors, which require periodic refreshing to maintain the stored data.
Recently, resistance variable memory elements, which include programmable conductor random access memory (PCRAM) elements employing a chalcogenide material, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. One such PCRAM device is disclosed in U.S. Pat. No. 6,348,365, assigned to Micron Technology Inc. and incorporated herein by reference. In typical PCRAM devices, conductive material, such as silver, is moved within the chalcogenide material to alter the cell resistance. Thus, the resistance of the chalcogenide material can be programmed to stable higher resistance and lower resistance states. The programmed lower resistance state can remain intact for a long period, typically ranging from hours to weeks, after the voltage potentials are removed.
One aspect of fabricating resistance variable memory cells, which also occurs in fabrication of other integrated circuit devices, involves contacts used for connecting the memory cells to integrated circuitry formed several layers beneath the cells. Oftentimes, because of the high aspect ratio of long vias, contacts provided therein have either sharp comers, keyholes or both, created during the contact formation. The sharp comers are created by the long, vertical sidewalls of the vias. During plug formation in vias having high aspect ratios, CVD is incapable of filling the vias completely before the plug is closed, resulting in keyhole defects in the plugs. Subsequent chemical mechanical polishing or etchback fabrication steps can expose the key and are to create a completely smooth via topography. Additionally, cell material can fall into the keyholes during deposition, leading to a non-uniform cell surface, or worse, a cracks or breaks in the cell surface, which can cause the cell to malfunction, and effectively limits the materials that can be used in the memory cell.
The sharp comers and/or keyholes may also result in inconsistent and unreliable switching of the memory device. Put another way, these problems make the cell unable to reliably switch between high and low resistance states. Such problems also reduce memory device yield and the lifetime of a memory cell is potentially cut short. Therefore, it is important in the fabrication of integrated circuit contacts, including those employing resistance variable memory cells, to create a smooth-surfaced or faceted conductive plugs on which subsequent material may be deposited.
Referring to FIG. 1, a cross sectional view of a portion of a conventional memory device 100 is shown. A dielectric layer 110 contains metal plugs 120 which connect to lower structures of the memory device 100. For purposes of clarity, only one plug 120 is shown. For a resistance variable memory, cell material 140 is formed over the dielectric 110 and plugs 120. As discussed above, however, keyholes 130 in the metal plugs 120 can prevent smooth deposition of the cell material 140 and cause interference with the operation of the memory cell 100. Keyholes 130 are very difficult to avoid when employing conventional methods of forming metal plugs 120, such as chemical vapor deposition (“CVD”).
Accordingly, there is a need for conductive contacts mitigate against formation of keyhole defects. There is also a need and desire for conductive contacts for use in a resistance variable memory device that compensate for keyhole and other defects.